1. Technical Field of the Invention
The present invention relates to an authentication technology of an individual voice.
2. Description of the Related Art
A conventional voice authentication technology has been suggested, which determines the authenticity of a person to be authenticated (hereinafter referred to as a “subject”) by comparing, with a threshold, the distance between a feature quantity of a voice previously obtained from an authorized user (hereinafter referred to as a “reference voice” or “registration voice”) and a feature quantity of a voice obtained from the subject (hereinafter referred to as a “sample voice” or “authentication voice”). Patent Reference Japanese Patent Application Publication No. 2003-248661 describes a configuration in which the threshold is changed according to the purpose of authentication or according to a required accuracy level of authentication.
FIG. 8 is a graph used to estimate voice authentication. A False Rejection Rate (FRR) in FIG. 8 means the probability (user rejection rate) that the authenticity of a subject will be rejected during authentication even though the subject is an authorized user, and a False Acceptance Rate (FAR) means the probability (stranger acceptance rate) that the authenticity of a subject will be accepted during authentication even though the subject is not an authorized user. As can be seen from FIG. 8, if the threshold used in authentication is set to a value “a” in FIG. 8, the probability of rejecting unauthorized subjects is maintained at a sufficiently high level while the probability of rejecting the authorized user is sufficiently reduced.
However, the distance between the authentication voice and the registration voice changes according to an ambient sound (hereinafter, referred to as “sample noise” or “authentication noise”) generated during authentication since the authentication noise affects the characteristics of the authentication voice. Accordingly, the curve of each of the FRR and the FAR in FIG. 8 moves parallel to the horizontal axis (distance) according to the characteristics of the authentication noise. For example, if the FAR shown by a solid line in FIG. 8 is changed to a dashed line L1, then the probability of erroneously accepting the authenticity of a stranger is increased (i.e., the accuracy level of authentication is lowered) and, if the FRR in FIG. 8 is changed to a dashed line L2, then the probability of rejecting the authenticity of the authorized user is increased (i.e., the convenience of authentication is lowered). That is, the conventional voice authentication has a problem in that the balance between the accuracy of authentication and the convenience of authentication is disrupted according to the characteristics of the authentication noise.
This problem cannot be solved even by changing the threshold according to the purpose of authentication or the required accuracy level as in Patent Reference Japanese Patent Application Publication No. 2003-248661. This problem is more serious when authentication is performed with a portable electronic device represented by a mobile phone since the characteristics of authentication noise are changed in various ways according to the use environment of the electronic device.